1. Field of the Invention
The present invention relates to a novel process for preparing 3-methyltetrahydrofuran, which is a useful substance as a comonomer of polytetramethylene ether glycol which is a raw material of elastic fibers called spandex fibers, or as a solvent for a specific use purpose.
2. Description of the Related Arts
As preparation methods of 3-methyltetrahydrofuran, various methods have heretofore been disclosed, and for example, there are a method which comprises the hydrogenation of citric acid (EP Disclosure No. 277562) and another method which comprises the hydrogenation of 4-hydroxy-2-methylbutane-1,2-epoxide (U.S. Pat. No. 3,956,318). However, these starting materials are less easily available, and therefore these methods are industrially impractical. In addition, a method which comprises the hydrogenation of methylmaleic acid or methylsuccinic acid (Japanese Patent Publication No. 9463/1974) has also been disclosed, but this starting material is scarcely available and what is worse, conditions for the hydrogenation are also severe. These inconveniences make its industrial practice difficult.
Furthermore, there is a method which comprises partially hydrogenating 1,4-butynediol to form 2-buten-,1,4-diol, hydroformylating and hydrogenating the thus formed 2-buten-1,4-diol to obtain 2-methyl-1,4-butanediol (U.S. Pat. No. 3,859,369), and then dehydrating/cyclizing this product in the presence of an acid catalyst to obtain 3-methyltetrahydrofuran. However, this method has some drawbacks. For example, the selectivity of 2-buten-1,4-diol by the partial hydrogenation of 1,4-butynediol is not sufficiently high, and the yield of the desired hydroformylation product of an internal olefin such as 2-buten-,1,4-diol is not sufficiently high, either.
In addition, a method has been disclosed which comprises the hydrogenation of .beta.-formyl isobutyrate to obtain 3-methyltetrahydrofuran or 2-methyl-1,4-butanediol (Japanese Patent Application Laid-open No. 219981/1994). .beta.-formyl isobutyrate which is a starting material in this method can be synthesized in a known manner such as the hydroformylation of a methacrylic acid ester [Bull. Chem. Soc. Japan, Vol. 50, p. 2351 (1977)], but the production of an .alpha.-isomer whose boiling point is close to that of .beta.-formyl isobutyrate is not avoidable. Thus, a large amount of energy is required for its separation, and for this reason, the disclosed method is not considered to be industrially advantageous.
Under such circumstances, the present invention has been developed to solve the problems in the above-mentioned various manufacturing methods of 3-methyltetrahydrofuran, and an object of the present invention is to provide a process for preparing 3-methyltetrahydrofuran by an industrially advantageous procedure.